[test] Use pytest instead of nosetests (#482)

`nosetests` is no longer being maintained : https://github.com/nose-devs/nose/issues/1099
and will stop working in py 3.10 as can be seen in #480

test/test_age_restriction.py

@@ -7,8 +7,7 @@ import sys
 import unittest
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
-from test.helper import try_rm
-
+from test.helper import try_rm, is_download_test
 
 from yt_dlp import YoutubeDL
 
@@ -32,6 +31,7 @@ def _download_restricted(url, filename, age):
     return res
 
 
+@is_download_test
 class TestAgeRestriction(unittest.TestCase):
     def _assert_restricted(self, url, filename, age, old_age=None):
         self.assertTrue(_download_restricted(url, filename, old_age))